Display systems can used to make a desired image visible to a user (viewer). Wearable display systems can be embodied in a wearable headset which is arranged to display an image within a short distance from a human eye. Such wearable headsets are sometimes referred to as head mounted displays, and are provided with a frame which has a central portion fitting over a user's (wearer's) nose bridge and left and right support extensions which fit over a user's ears. Optical components are arranged in the frame so as to display an image within a few centimetres of the user's eyes. The image can be a computer generated image on a display, such as a micro display. The optical components are arranged to transport light of the desired image which is generated on the display to the user's eye to make the image visible to the user. The display on which the image is generated can form part of a light engine, such that the image itself generates collimated lights beams which can be guided by the optical component to provide an image visible to the user.
Different kinds of optical components have been used to convey the image from the display to the human eye. These can include lenses, mirrors, optical waveguides, holograms and diffraction gratings, for example. In some display systems, the optical components are fabricated using optics that allows the user to see the image but not to see through this optics at the “real world”. Other types of display systems provide a view through its optics so that the generated image which is displayed to the user is overlaid onto a real world view. This is sometimes referred to as augmented reality.
Waveguide-based display systems typically transport light from a light engine to the eye via a TIR (Total Internal Reflection) mechanism in a waveguide (light guide). Such systems can incorporate diffraction gratings, which cause effective beam expansion so as to output expanded versions of the beams provided by the light engine. This means the image is visible over a wider area when looking at the waveguide's output than when looking at the light engine directly: provided the eye is within an area such that it can receive some light from substantially all (i.e. all or most) of the expanded beams, the whole image will be visible to the user. Such an area is referred to as an eye box.
In one type of head mounted display, the frames support two light engines, which each generate an image for a respective eye, with respective guiding mechanisms which each guide the image to project it at a proper location with respect to the associated eye so that the wearer's eyes operate in unison to receive a single non-distorted image.